A brushless motor includes a permanent magnet rotor and a stator made up of a certain number of windings, typically three, usually connected in a star configuration. However, the number of windings may also be connected independently from each other. Both terminals of each phase winding may be available for external connection to permit the driving of each winding in a completely independent manner from the other windings.
Another common configuration that may be used is the so-called delta or polygonal configuration. Referring by way of example to a traditional triphase brushless motor connected in a star or delta configuration, the driving generally occurs using integrated circuits. Output power stages of the integrated circuit that drives the phase windings may be implemented in the form of a full-wave triphase bridge stage made up of six bipolar or MOS power transistors.
FIG. 1 represents a typical driving stage, and the scheme of a triphase DC brushless motor for the two most common configurations. The prevalent driving method for this type of motor is the bipolar method. That is, at any instant two of the windings are fed and the third winding is unpowered. For example, the third winding is unpowered by placing the respective output node of the full-bridge or half-bridge high impedance state.
The windings are switchably driven according to a duty cycle sequence which must be synchronized with the instantaneous position of the rotor, i.e., the position of its magnetic axis. During a bipolar driving mode, the position of the rotor may be easily monitored by monitoring the back electromotive force (BEMF) of the winding that is momentarily unpowered during a certain switching phase, or the rotor position may be more conventionally sensed through physical position sensors, like Hall-effect sensors.
An alternative and advantageous driving method, described in European Patent Application No. 96,830,295.0, which is assigned to the assignee of the present invention, is based on applying a driving signal to all the windings in a continuous mode and with a waveform profile that optimizes motor performances. For instance, the waveform profile are sinusoidal drive signals. With this driving method, it is advantageous to use position sensors without purposely interrupting the driving to monitor the BEMF.
At present, there exist on the market motors with a number of position sensors equal to the number of phases of the motor itself. However, polyphase DC brushless motors with only one sensor to reduce costs should be available in the future. This will reduce costs, but will provide for an angular resolution of the positions of the rotor with a much lower resolution than the angular resolution compatibilities of the driving systems that are presently commonly used. For instance, a single Hall effect sensor may discriminate positions with an angular resolution of 180 degrees. In contrast, a common driving system for polyphase motors is far more precise by having an angular resolution much higher than 180 degrees.
A specific start-up procedure is required for a Brushless DC motor with respect to sensorless motors. A specific start-up procedure is also required in cases where the information obtained from the sensors is insufficient due to an insufficient angular resolution for identifying the right phase to be commanded, and for yielding the maximum torque.
In these cases, a start-up routine must be performed to generate the torque necessary to overcome static friction, and to bring the motor to rotate at a speed sufficient to permit exploitation of monitoring the voltage induced on the windings. This is through which momentarily is not forced any driving current by the rotor (BEMF voltage), or the signal(s) originating from the position sensors to synchronize the phase switching.